1. Technical Field
The invention relates to a color-separation optical system for separating incident light into light of plural colors and an imaging apparatus having the color-separation optical system.
2. Description of the Related Art
In general, an imaging apparatus such as a television camera and a video camera includes a color-separation optical system. FIG. 52 shows the configuration of a color-separation optical system of related art. The color-separation optical system 101 separates incident light L entering through a taking lens 102 into three light components, that is, blue light LB, red light LR, and green light LG. Imaging devices for the respective color light 4B, 4R, and 4G, such as CCDs (Charge Coupled Device), are disposed to correspond to the respective color light separated by the color-separation optical system 101. The color-separation optical system 101 is one called the Philips type color separation system. The color-separation optical system 101 includes a first prism 110, a second prism 120 and a third prism 130 in order from the light-incident side along an optical axis Z1 and is configured to extract the blue light LB by the use of the first prism 110, to extract the red light LR by the use of the second prism 120, and to extract the green light LG by the use of the third prism 130.
A blue-light reflecting dichroic film DB1 is formed on a reflecting/transmitting surface 111 of the first prism 110. A red-light reflecting dichroic film DR1 is formed on a reflecting/transmitting surface 121 of the second prism 120. A trimming filter 151 is disposed on a light-exiting surface of the first prism 110. A dichroic film 151A is formed on a light-exiting surface of the trimming filter 151. Similarly, a trimming filter 152 having a dichroic film 152A formed thereon is disposed on a light-exiting surface of the second prism 120, and a trimming filter 153 having a dichroic film 153A is formed thereon is disposed on a light-exiting surface of the third prism 130. The trimming filters 151, 152, and 153 are provided to bring a spectral characteristic closer to an ideal characteristic and have a role of adjusting spectral characteristics of wavelength components, which have not been adjusted satisfactorily by the blue-light reflecting dichroic film DB1 and the red-light reflecting dichroic film DR1.
FIG. 54 shows a spectral characteristic, which is generally considered as ideal in a color imaging apparatus, in terms of three colors of red (R), blue (B), and green (G). The ideal characteristic shown in FIG. 54 is normalized so that the maximum values of the respective light components are 1. The “ideal characteristic” is converted from chromaticity coordinates of three primary colors of a color reproducing medium and can be obtained by a linear transformation of the color-matching function in the XYZ color coordinate system. Here, the “color reproducing medium” is one that reproduces (displays) an image captured by an imaging apparatus and means a display device such as a liquid crystal monitor or a projector. FIG. 53 shows an example of the chromaticity coordinates of three primary colors R, G, and B for obtaining the ideal characteristic. The three primary colors R, G, and B determine a color range that can be reproduced by the color reproducing medium.
When the same characteristic as the ideal characteristic shown in FIG. 54 is obtained by using the color-separation optical system 101 shown in FIG. 52, it is possible to reproduce colors ideally. However, in practice, it is difficult to completely obtain the same characteristic as the ideal characteristic and thus, it is designed to obtain a characteristic approximated to the ideal characteristic. In the color-separation optical system 101 of the related art, it is designed to obtain a characteristic approximated to the ideal characteristic by properly adjusting the dichroic films DB1 and DR1 formed in the prisms and the dichroic films 151A, 152A, and 153A formed in the trimming filters 151, 152, and 153. FIG. 55 shows a spectral transmission characteristic of the color-separation optical system of the related art, which is designed in such a way.
FIG. 56 shows a design example of the dichroic films DB1 and DR1 used in the color-separation optical system 101 of the related art. As shown in FIG. 56, dichroic films having such a characteristic that the transmissivity characteristic curve of the wavelength band goes up or down more sharply than the ideal characteristic curve shown in FIG. 54 were used in the related art as the dichroic films DB1 and DR1. Unnecessary wavelength components of light exiting from the exiting surfaces of the prisms were intercepted by the use of the trimming filters 151, 152, and 153 having the dichroic films 151A, 152A, and 153A, respectively.
In this way, the characteristics were adjusted by the use of various trimming filters. For example, JP 2005-208256 A has proposed a method of improving color reproduction by enhancing a brightness level of a skin color by the use of a trimming filter having a special spectral transmission characteristic. A method of adjusting the transmission characteristic by disposing a half mirror on the bonding surface between the second prism 120 and the third prism 130 instead of the dichroic film DR1 and forming dichroic films having a transmission characteristic approximated to the ideal characteristic in the trimming filters 152 and 153 was also known. FIG. 57 shows a spectral characteristic of a color-separation optical system of related art which is made to be approximated to the ideal characteristic by making such a special adjustment.
However, the color-separation optical system of the related art using a trimming filter having a dichroic film in the exiting surface of a prism has a wavelength range in which reflectivities are high wavelength-selectively, as a characteristic of the dichroic film. Therefore, multiple reflections occur between the dichroic surface and the imaging surface to cause ghost and flare, thereby deteriorating image quality. FIG. 58 shows an example of the multiple reflections occurring in the exiting surface of the third prism 130 for extracting the green light LG in the color-separation optical system 101 of the related art. As shown in FIG. 58, the imaging device 4G includes an imaging surface 401G, a cover glass 402, and an external electrode 403. For example, a part of the green light LG passing through the green trimming filter 153 is reflected by the imaging surface 401G and the returning light is reflected in accordance with the wavelength selection characteristic of the dichroic film 153A of the trimming filter 153. In this way, the multiple-reflected light 160 is generated, which causes ghost and flare. Accordingly, it was difficult to embody an imaging apparatus having the ideal spectral characteristic with ghost and flare being suppressed.
FIG. 59 shows the configuration of another color-separation optical system of related art. The color-separation optical system 201 is one called a Philips type color-separation optical system. The color-separation optical system 201 includes an IR (infrared) cut filter 103, a first prism 110, a second prism 120, and a third prism 130 in order from the light-incident side along an optical axis Z1, and is configured to extract the blue light LB by the use of the first prism 110, to extract the red light LR by the use of the second prism 120, and to extract the green light LG by the use of the third prism 130.
A blue-light reflecting dichroic film DB1 is formed on a reflecting/transmitting surface 111 of the first prism 110. A red-light reflecting dichroic film DR1 is formed on a reflecting/transmitting surface 222 of the second prism 120. The first prism 110 and the second prism 120 are disposed so that the surface 111 having the blue-light reflecting dichroic film DB1 formed thereon and the light incidence surface 221 of the second prism 120 face each other with an air gap 110AG interposed therebetween. An antireflection film AR1 for reducing the reflection of the incident light is formed on the incidence surface 221 of the second prism 120.
In the color-separation optical system 201 shown in FIG. 59, since the first prism 110 and the second prism 120 are disposed with the air gap 110AG interposed therebetween, reflection occurs by the air interface between the two prisms, as shown in FIG. 60, which may cause unnecessary ghost and flare and deteriorate image quality. This phenomenon is caused by light that passes through the blue-light reflecting dichroic film DB1 and is then reflected by the incident surface 221 of the second prism 120. Here, since the blue light is reflected by the blue-light reflecting dichroic film DB1, the blue light hardly reaches the incidence surface 221 of the second prism 120. Since the red light is reduced by the IR cut filter 103 disposed in the earlier stage, the red light is relatively small. Accordingly, in the prism arrangement shown in FIG. 59, the most components of light reaching the incident surface 221 of the second prism 120 would be the green light. However, one for reducing reflectivities all over the visible band as shown in FIG. 48A is used as the antireflection film AR1, and the reflection of light particularly in the green wavelength range (about 500 nm to 600 nm) is not suppressed sufficiently. Accordingly, a part of light in the wavelength range is easily reflected by the incident surface 221 of the second prism 120. As shown in FIG. 60, of the reflected light, the light L1 passing through the blue-light reflecting dichroic film DB1 and entering again the first prism 110 enters a blue imaging device 4B to form an image. Thereby, the thus-formed image is observed as a double image with respect to an image formed by the normal blue light LB. Of the reflected light, the light L2 reflected by the surface 111 of the first prism 110 and entering again the second prism 120 enters the third prism 130 through the red-light reflecting dichroic film DR1 (because the light in the green wavelength range is abundant as described above) and finally enters a green imaging device 4G to form an image. Thereby, the thus-formed image is observed as a double image with respect to an image formed by the normal green light LG. As described above, since the antireflection film AR1 is formed on the incident surface 221 of the second prism 120 but the characteristic for reducing the reflectivities thereof is not sufficient, the ghost and the like occurs due to the reflection by the air interface between the two prisms.